1. Field of the Invention
The present invention relates to a plasma display apparatus including a plasma display panel.
2. Description of Related Art
In recent years, with the increase in screen size of display apparatuses, the demand for thin shape display devices is increasing and various kinds of thin display device have been put into the practical use. A plasma display panel of an alternating current discharge type has attracted much attention as one type of the thin shape display device.
FIG. 1 is a view schematically showing the construction of a plasma display apparatus equipped with such a plasma display panel.
In FIG. 1, PDP 10 as a plasma display panel is provided with m of column electrodes D1 through Dm and respective n of row electrodes X1 through Xn and row electrodes Y1 through Yn aligned to intersect with the respective column electrodes. The row electrodes X1 through Xn and the row electrodes Y1 through Yn, constitute a 1-th display line through an n-th display line in PDP 10 by respective pairs of row electrodes Xi (1xe2x89xa6ixe2x89xa6n) and Yi (1xe2x89xa6ixe2x89xa6n). The PDP 10 is constructed in such a way that discharge spaces enclosing a discharge gas are formed between the column electrodes D and the row electrodes X and Y and discharge cells constituting pixels are formed. at intersecting portions of the respective row electrode pairs and the column electrodes including the discharge spaces.
In this case, the respective discharge cell has only two states of xe2x80x9clight emissionxe2x80x9d and xe2x80x9cno light emissionxe2x80x9d, since light is emitted by utilizing a discharge phenomenon. That is, the PDP 10 is capable of displaying only brightness of two gray scales of lowest brightness (non light emitting state) and highest brightness (light emitting state).
Hence, a driver 100 carries out a gray-scale drive using the subfield method for the PDP 10 in order to realize the display with halftone brightness in accordance with an inputted image signal.
According to the subfield method, an inputted image. signal is converted into, for example, corresponding 4 bit pixel data for each of the pixels. In correspondence respectively with the bit digits of the four bits, 1 field is constituted by four subfields SF1 through SF4 as shown in FIG. 2.
FIG. 3 is a diagram showing various drive pulses applied by the driver 100 on the row electrodes and the column electrodes of PDP 10 and application timings thereof in one subfield.
First, at simultaneous resetting step Rc, the driver 100 applies reset pulses RPX having a positive polarity simultaneously to the respective row electrodes X1 through Xn and applies reset pulses RPY having a negative polarity simultaneously to the respective row electrodes Y1 through Yn as shown in FIG. 3. In accordance with application of the reset pulses RPX and RPY, all of the discharge cells of PDP 10 are discharged to reset. After finishing the reset discharge, a predetermined amount of wall charge is uniformly formed in the respective discharge cells and the wall charge is maintained.
By executing the simultaneous resetting step Rc, all of the discharge cells in PDP 10 are initialized to the state (sustaining discharge) capable of emitting light in a light emission sustaining step Ic, mentioned later (hereinafter, referred to as xe2x80x9clight emitting cellxe2x80x9d state).
Next, at a pixel data writing step Wc, the driver 100 separates respective bits of the 4 bit pixel data in correspondence with the respective subfields SF1 through SF4 and generates pixel data pulses having a pulse voltage in accordance with the logical level of the corresponding bit. For example, at pixel data writing step Wc of the subfield SF1, the driver 100 generates a pixel data pulse having a pulse voltage in accordance with the logical level of the first bit of the pixel data. In this process, the driver 100 generates a pixel data pulse having a pulse voltage of high voltage when the logical level of the first bit is xe2x80x9c1xe2x80x9d, or low voltage (0 volt) when the logical level of the first bit is xe2x80x9c00xe2x80x9d. Further, the driver 100 applies the pixel data pulses successively to the column electrodes D1 through Dm as shown in FIG. 3 as a group of pixel data pulses DP1 through DPn for respective single display line in correspondence with each of the 1-th through the n-th display lines. Further, the driver 100 generates a scan pulse SP having a negative polarity as shown in FIG. 3 in synchronism with an application timing of each of the respective pixel data pulse group DP and applies it successively to the row electrodes Y1 through Yn. With this operation, there causes discharge (selective erasure discharge) only at the discharge cell at a portion intersected with the display line applied with the scan pulse SP and xe2x80x9ccolumnxe2x80x9d applied with the pixel data pulse having high voltage. By the selective erasure discharge, wall charge held in the discharge cell is extinguished and the discharge cell is shifted to a state of being incapable of emitting light (sustaining discharge) in the light emission sustaining step Ic, mentioned later (hereinafter, referred to as xe2x80x9cno light emitting cellxe2x80x9d). Meanwhile, the selective erasure discharge is not caused in the discharge cell applied with the pixel data pulse having low voltage even when the discharge cell is applied with the scan pulse SP and the discharge cell maintains the state of being initialized at the simultaneous resetting step Rc, that is, the state of xe2x80x9clight emitting cellxe2x80x9d.
That is, according to the pixel data writing step Wc, the respective discharge cell of PDP 10 is set to either of the xe2x80x9clight emitting cellxe2x80x9d state and the xe2x80x9cno light emitting cellxe2x80x9d state in accordance with the pixel data based on the inputted image signal.
Next, at the light emission sustaining step Ic, as shown in FIG. 3, the driver 100 applies sustaining pulses IPX having a positive polarity and sustaining pulses IPY having a positive polarity respectively to the row electrodes X1 through Xn and the row electrodes Y1 through Yn alternately repeatedly. Further, the number of times (periods) of application of the sustaining pulses IPX and IPY in one subfield are set in accordance with weighting of the respective subfields as shown in FIG. 2. In this process, only the discharge cell at which wall charge is present, that is, the discharge cell brought into the xe2x80x9clight emitting cellxe2x80x9d state, carries out a sustaining discharge each time the sustaining pulses IPX and IPY are applied. That is, only the discharge cell set to the xe2x80x9clight emitting cellxe2x80x9d state in the pixel data writing step Wc, repeats light emission in accordance with sustaining discharge by the number of times set in correspondence with the weighting of the respective subfield as shown in FIG. 2, and maintains the light emitting state.
The driver 100 carries out the above-described operation for the respective subfield. The brightness of an intermediate tone in correspondence with the image signal is expressed by a total number of light emission (in one field) associated with the sustaining discharge created in the respective subfield. That is, by the light emission associated with the sustaining discharge, an image in correspondence with the image signal is displayed.
However, according to the above-described driving operation utilizing the discharge phenomenon, discharges accompanied by light emission which are not related to the display image, that is, the resetting discharge and selective erasure discharge must also be produced. Particularly, as a result of the reset discharge, all of the discharge cells simultaneously emit light. Therefore, there arises a problem that, when displaying a black image or an image having a extremely low brightness near to the black peak, a deterioration in contrast becomes remarkable.
The invention has been made in view of the above-described problem and it is an object of the present invention to provide a method of driving a plasma display panel and a plasma display apparatus capable of preventing a deterioration of contrast in displaying an image having low brightness.
According to one aspect of the invention, there is provided a method for driving a plasma display panel in accordance with an image signal, said plasma display panel having a plurality of discharge cells constituting display pixels arranged in a matrix form, the method comprising: a simultaneous resetting step for applying reset pulses having gradual level changes at front edge portions thereof to each of the discharge cells to cause a reset discharge for initializing the respective discharge cells to either of a light emitting cell state and a non light emitting cell state; a pixel data writing step for applying a scan pulse for causing selective discharge to the-respective discharge cells to shift the discharge cells selectively to the non light emitting cell state or the light emitting cell state in accordance with pixel data corresponding to the image signal, and a-light emission sustaining step of applying to each of the discharge cells sustaining pulses for causing sustaining discharge for causing only the discharge cells brought into the light emitting cell state to emit light repeatedly, wherein the simultaneous resetting step includes a reset pulse waveform adjusting step of adjusting a time period before the level at the front edge portion of the reset pulse reaches a predetermined level, in accordance with an average brightness level of the image signal.
According to another aspect of the invention, there is provided a plasma display apparatus for driving a plasma display panel having capacitive discharge cells constituting display pixels arranged in a matrix form in accordance with an image signal, the apparatus comprising a reset pulse generating part for generating reset pulses for creating reset discharge for initializing each of the discharge cells to either of a light emitting cell state and a no light emitting cell state, a light emission driving part for selectively shifting the discharge cells to the non light emitting cell state or the light emitting cell state in accordance with the image signal and causing only the discharge cells brought into the light emitting cell state to emit light repeatedly, and an average brightness level measuring part for measuring an average brightness level of the image signal, wherein the reset pulse generating part comprises a power source for generating direct current power source voltage having a voltage value the same as a voltage value of pulse voltage in the reset pulse, a part for generating the reset pulse by applying the direct current power source voltage to the respective discharge cells via resistors, and a reset pulse waveform adjusting part for adjusting time constants of C-R circuits each comprising the discharge cell as a capacitive load and the resistor in accordance with the average brightness level.